Seems like a pretty clear case of caveat emptor

In the first place, the game is 30 years old. If the world had a sane copyright regime, it would already be in the public domain. Nobody should be particularly entitled to buying abandoned 30-year-old IPs and squatting on them to collect rent. All the more so when there would be no rent to collect if not for the derivative work being literally the only thing keeping the IP alive.

But let's suppose I am Atari and I have for some reason proceeded with buying said abandonware without doing my research. Upon discovering OpenTTD, I would hire the guy behind OpenTTD to work on a commercial version, keeping OpenTTD free to play but perhaps with some cool monetized expansion pack that would not have been possible without giving the developer the funding they need to work on it. That way I am making an investment in actually adding value to the game, and rewarding the person who kept it alive and in turn earning community goodwill, instead of investing in a shortsighted attempt to collect rent that backfires massively.

I go "oops I probably should have realised that existed before I decided to purchase the rights to and re-release a 999999 year old game which already has a GPL clone/spiritual successor/something".

I then go "well why re-release this ancient game running in an emulator, when this exists?" and ask the core team of OpenTTD if they want to monetize their steam/GOG releases now that I can licence out the TTD IP to them and remove any remaining legal ambiguity (and recoup my """investment""" via revenue sharing).

And if they don't I take it as a learning experience (to do my homework before I buy IPs) and release my TTD-in-an-emulator on steam and GOG knowing full well that its probably not going to generate many sales. Maybe I add "hey just so you know there's this really cool modern source port you can get for free..." to the description and hope that I can generate some sales off of good boy points.